An epicyclic conventionally comprises an internal sun gear and an external sun gear which are coaxial, with the internal sun gear being movable in rotation about its axis, with the external sun gear being stationary, with at least one planet gear being mounted to be movable in rotation on a planet carrier and meshing with both the internal sun gear and the external sun gear, with the planet carrier being able to pivot about the axis of the internal sun gear and the external sun gear. The inlet is typically formed by the internal sun gear and the outlet is formed by the planet carrier. The outer planet is also called an orbit gear.
In a turbomachine, epicyclic reduction gears are used in particular as speed reducers for reducing the speed of rotation of the fan rotor, regardless of the rotational speed of the turbine.
The document EP 1703174 describes such an epicyclic reduction gear, wherein the sprocket wheels forming the planet gears are mounted on pivots of the planet carrier by means of journal bearings. In other words, the planet carrier comprises cylindrical pivots engaged in the cylindrical holes of the planet gears. The reducer further comprises an oil supply passage opening at the interface between said cylindrical surfaces. In operation, a layer of oil must be present at the interface, in order to prevent seizing.
The journal bearings are generally lighter, less bulky and more reliable than bearings using rolling elements and they have a almost infinite service life, provided they are constantly supplied with oil and the oil contains no abrasive particle.
In case of failure in the oil supply circuit, for example in case a pump failure, the oil supply to the journal bearing must be maintained long enough to start an auxiliary pump or to stop the turbomachine, for example. This period amounts, for example to several tens of seconds.
For this purpose, the document EP 1703174 provides for the forming of accumulators in the planet carrier, with each accumulator being able to supply oil to a journal bearing in case of failure, for a given duration. The structure of such accumulators and the locations thereof make the production of the planet carrier difficult and increase the dimensions and the mass thereof.
Furthermore, controlling the oil flow supplied to the journal bearing in case of breakdown is relatively difficult. In particular, the flow rate should be relatively low so as to be able to supply oil for a sufficiently long time, while being sufficient to prevent seizing.
A passage with a small diameter is generally used to limit a fluid flow rate. In this application, however, a passage having a sufficiently small diameter over a relatively long length (e.g. a length of approximately 100 times the diameter of the passage) is not only difficult to realize but it is also subject to clogging by particles contained in the oil circuit. Another solution would be to use a strainer at the inlet of the passage so as to limit the flow rate but in this case too, a risk of clogging of the strainer by the particles exists.